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Geothermal Earth Loop Options Is there a single “best” approach?
An overview of the different types of geothermal loops and
their various cold climate adaptations
PRESENTER: C. Mark Sakry, CGD
Northern GroundSource Inc. www.NorthernGroundSource.com
1. The Geothermal Heat Pump Concept INTRODUCTION: The Simple Logic of a “Ground Source” Approach
Recommended References
Ground Source Heat Pump Residential and Light Commercial Design and Installation Guide; Remund et. al, International Ground Source Heat Pump Association, Oklahoma State University, Stillwater, 2009.
Ground-Source Heat Pumps: Design of Geothermal Systems for Commercial and Institutional Buildings; Kavanaugh and Rafferty, American Society of Heating, and Air-Conditioning, Inc., 1997.
A common air conditioner is a simple Air-to-Air Heat Pump—exchanges 74°F indoor air with outdoor temperatures that fluctuate broadly and can often swing to over 100°F.
An air source heat pump adds Air-to-Air heating capability—it exchanges 70°F indoor air with outdoor temperatures that can swing well below 20°F (common ASHP operating range limit).
Where might we find temperatures close-by that are far more stable and efficient?
High Grade Geothermal Energy
Old Faithful Geyser “Hot Rocks” Power
Low Grade Geothermal Energy
“Solar” Geothermal GeoExchange Systems
Surface ground temperature swing is much narrower than seasonal air tempera-ture above and gradually becomes stable.
DULUTH SOIL STATISTICS: MEAN SURFACE TEMPERATURE 1 ft. = 41°F ANNUAL SWING = 28°F DAYS TO MINIMUM = 37 DEEP EARTH TEMPERATURE = 48°F
–20 –10
41°F
+10 +20
4 8 12 16 20 24 28
depth, ft
ground temperature, °F
average soil (0.6 ft²/day or 0.025 ft²/hr)
48°F
0
55°F 27°F
48°F 34°F
Stable
Heat pump technology logically goes…underground!
11
Load to Source: Sustainability Factor
9 kWh T Power Plant 2.7 kWh E
Ground Source Heat Pump COP = 3.7
10 kWh T
GSHP requires only 9 kWhT from the source to provide 10 kWhT to the building since they can provide a COP of 3.7
Generation and Transmission Losses
Source Load
Environmental Benefits—Great!... But Tend to Be Geographically Specific
GSHPs create no direct emissions— BUT… Like other electric technologies, GSHPs create “upstream” carbon emissions… Entirely dependent on the method of power generation in the area [NOTE: Fossil fuels represented on this chart are “point of use” only!]
50,000 BTUH Heating Load (Typical) Duluth, Minnesota
Economic Benefits—Vary depending on Energy Cost Savings per Net System Cost
Savings compared to WHAT?... System Cost com-pared to WHAT?... Cost of Ownership = Total System Design & Installation Cost…
--PLUS-- System Operating & Mtce. Cost Over Time …COMPARED TO SOMETHING ELSE!
50,000 BTUH Heating Load (Typical) Duluth, Minnesota
($1,127)
($3,862)
($2,284)
($1,364)
$3,571 $3,748
(NOTE: Rate Changes Reflected Since 2011)
2013 Geothermal Rebate Programs Regional Northeastern Utilities
Minnesota Power $200/T Closed Loop ($100/T Open Loop) + $200 ECM
[BONUS: Now Through 08/31/2013 … $50/T GSHP + $50 ECM] Great River Energy Affiliates:
Cooperative Light & Power Lake Country Power East Central Energy Arrowhead Electrical Cooperative $400/T Closed or Open Loop + $100 ECM
Consider that GSHP installation costs and benefits are scalable!
Common Misconceptions About Geo’
Take 50°F from the ground and deliver it to the house, then make up the remaining “difference” with supplemental heat
Geothermal heat pumps are not well suited for extremely cold climates—you just don’t get enough benefit from them
Simply replaces a conventional furnace or boiler…and everything else remains equal
2. Understanding the Technology How Geothermal Heating & Cooling Works (in 3½ minutes or less!)
Ground Source Heat Pumps
Water to Air GHP (Forced Air)
Water to Water GHP (Hydronic)
Simple Heat Pump Mechanical energy from the human is used to compress air inside the tube of a simple tire pump.
Simple Heat Pump Mechanical energy from the human is used to compress air inside the tube of a simple tire pump. Air inadvertently gets hot as it is compressed to higher pressure.
Simple Heat Pump Mechanical energy from the human is used to compress air inside the tube of a simple tire pump. Air inadvertently gets hot as it is compressed to higher pressure. In process of pumping air… heat is also being pumped.
Introducing: The Compressor The Compressor is the GSHP’s primary working unit where gas is compressed, heated, and “pumped” to its heat exchange delivery point. A refrigerant gas (with much better heat concentrating properties than air) is used. SCROLL: TOP VIEW
SCROLL COMPRESSOR
Zeroth Law of Thermodynamics
WIKIPEDIA: When two systems…are brought in diathermic contact with each other they exchange heat to establish a thermal equilibrium between each other. SIMPLE INTERPRETATION: Heat moves to Cold…Always!
Typical Forced Air GSHP System
Air Loop
Ground Loop Refrigerant Loop
Domestic Hot Water Loop
Forced Air GSHP Heating Cycle
Forced Air GSHP Cooling Cycle
LK
QUESTIONS?
3. GSHP System Design There are many variables to consider in GSHP system design.
The “geothermal system” is generally assigned to the “supply” side of heating/cooling functions.
Supply Side (GSHP) Delivery Side (HVAC)
Ground Heat Exchanger (GHEX)
Ground Source Heat Pump (GSHP or GHP)
Loop Pump or Flow Center
Some Peripheral and Auxiliary Components (incl. Controls)
Duct System (incl. Air Exchanger)
Radiant Floor Tubing, Manifolds, Zone Pumps and Controls
Radiant Baseboards, Panels, Radiators
Plumbing/Piping Delivery Systems
Many GSHP/HVAC Considerations (Owner must become “educated” & involved!)
Many GHEX/Earth Loop Options (Who decides?... Who designs?)
Design Procedure Estimate Heating/Cooling loads (BTUH) Determine proper heat pump size(s) Select indoor air/water distribution system(s) Estimate the building’s energy requirement Estimate the ground heat exchanger loads
--Annual load --Design month load
SIMPLE FORCED AIR EXAMPLE: 2,500 to 3,000 ft2 House (Typical)
DULUTH HOUSE: New Construction DULUTH INTL AP (Bin Weather Data) 50,000 BTUH Heat Loss (@ -20°F OAT) 24,750 BTUH Heat Gain (@ +84°F OAT) 5 Ton 3HT/2CL Forced Air Heat Pump
[NOTE: 1 Ton = 12,000 BTUH] Compare to LP or NG Furnace w/AC
Simple Rule #1: As it gets colder, the rate of structural heat loss increases.
PEAK HEATING LOAD per “DESIGN DAY” Design to the most “reasonable” lowest outdoor temperature. FOR EXAMPLE: Duluth INT ≈ -20°F THAT MEANS: If the indoor tempera-ture is kept at 70°F… That is a difference of 90°F between indoor and outdoor temperatures. Figure 3.6 Weather Bin Data for Grand Forks, ND
0 BTUH
- 50,000 BTUH (Current House @ -20F)
+24,750 BTUH
Auxiliary Electric Resistant Heat… Supplemental/Emergency Heat “Option”
PROPER USE: • Brief intermittent
“heat boost” for extreme cold periods (below balance point)
• Occasionally neces-sary GSHP/GHEX system sizing scale-back (site limitations)
• Availability of optional “emergency” heat
• Make up for under-sized or inadequate GSHP installation—after the fact!
CURRENT EXAMPLE: 5kW Supplemental 10kW Emergency
1kW = 3,414 Btu
Simple Rule #2: Lower loop temp. = less GSHP heat output…but also less loop cost!
GV/GH 580/581
30°F EWT suggested for 5T Horz. Earth Loop Designed to 8’ Depth
PROPER GSHP SIZING per “PEAK LOAD” Design to the most reasonable minimum loop temperature. FOR EXAMPLE: NE Minnesota ≈ 30°F NW Wisconsin ≈ 30°F RESULT: Reduces GHEX Installation Cost! Remember Goal—maximize benefits… while minimizing cost!
51,700 BTUH
61,700 BTUH
52,900 BTUH
[Current 5 Ton 3HT/2CL Forced Air GSHP]
LOOP TEMP.
Winter Earth Temp at 8’ Depth
Not Possible!
DESIGN LOOP TEMPERATURE
Antifreeze protection is required for lower loop temperatures (i.e., Leaving Water Temperature).
51,700 BTUH GSHP CAP @ 30F EWT 50,000 BTUH Heating Load @ -20°F
5T GSHP Runtime & Operating Cost 4T
41,100 BTUH GSHP CAP @ 30F EWT 50,000 BTUH Heating Load @ -20°F
4.0<5T GSHP Runtime & Operating Cost
Similar Result?...What’s the Hitch?
51,700 BTUH GSHP CAP $831.82 Annual Heating Cost
41,100 BTUH GSHP CAP $836.67 Annual Heating Cost
Saves Only 272 ft. of Total Pipe GHEX Sizing (i.e., cost)…about the same!
5,068 ft. Total Pipe 4,796 ft. Total Pipe
Saves 26 ft. Off Each Bore Hole (Slightly More Savings on Drilling)
4 X 250 ft. Total Bore 4 X 224 ft. Total Bore
GSHP Sizing Rules of Thumb
Cooling-Dominant Derived Rule: Do not
size GSHP heating capacity more than 25% above the cooling load (impractical?)
Manufacturer-Derived Rule: Do not size GSHP to less than 85% of the peak heating load (roughly 1 Ton undersizing)
Cold Climate Sizing Trend: Size to perform 96-100% of all the heating (but consider sizing scale-back for honest “hardships”)
QUESTIONS?
4. Earth Loop Considerations are Site Specific Is there a single “best way” to do it?...There are many options!
Common GHEX/Loop Options
GHEX Options by General Descending Cost:
Vertically Bored in Rock Vertically Bored “Standing Column” (Rock) Vertically Bored in Deep Soil/Overburden Horizontally Drilled Horizontally Trenched or Excavated Lake Loop (Permit?) Pond Loop (Existing Pond) Open Loop / Pump & Dump
Common GHEX/Loop Options
Wetlands Wetland concerns must be evaluated on every site… prior to breaking ground. It may not be completely evident that an area is actually classified as type of wetland until inspected by the proper authority. Required Sequencing:
• AVOID • MINIMIZE • MITIGATE
Well Logs Well logs can provide excellent information about local geological formations and soil types. MN County Well Index:
http://www.health.state. mn.us/divs/eh/cwi/ Site test boring data can also be very useful!
Soil Properties & Composition Site soil samples can be cross referenced with a soil thermal properties table for more exacting ranges of conductivity. Bore hole cutting samples can also be collected and analyzed for composition and thermal properties.
Conductivity Testing An in situ (on site) thermal conductivity test is the most effective way to determine the thermal properties of soil and rock across a specific formation.
It can be well worth the expense on larger boring projects where margins in cost are critical. It must be done at the design stage!
High Density Polyethylene (HDPE) has become widely established as the pipe of choice across the GSHP industry. It possesses the best characteristics of durability and conductivity…combined.
HDPE Pipe Joining Requires Heat Fusion
HDPE Pipe Joining Requires Heat Fusion
The Mother of All Fusion Rigs!
Header Configuration for 8T Vertical GHEX Loop flow is balanced using a “Step-down” Reducing Reverse Return design—system air purging and flushing is achieved by “choking” flow progressively along header as fluid is channeled through loops and available flow decreases.
Wall and Slab Penetrations Should be completed before pouring!
IGSHPA STANDARD 3A.11 (1996) All pipes passing through walls will be sleeved and sealed with non-hardening caulking material [emphasis added].
SCH40 PVC Pipe Sleeves (Typical)… High thermal resistance (insulating value), rigidity, and excellent adherence to sealing caulks, foams, and hydraulic cements.
Wall and Slab Penetrations Should be completed before pouring!
IGSHPA STANDARD 3A.11 (1996) All pipes passing through walls will be sleeved and sealed with non-hardening caulking material [emphasis added].
Long Sweep Elbows for Pipe Bends!… Proper pipe sleeve sizing and configuration will make it a lot easier to place geothermal HDPE supply/return header pipes later.
Closed-Loop Vertically Bored GHEX
Nominal versus Design Pipe Length REMEMBER: Maximum Performance Benefit…at Minimum Installation Cost
Nominal (Fixed) Length Design Loop Length Generally acceptable
for smaller (1 to 16 Ton) GSHP systems where designer is very familiar with local site geology and climate.
Design/performance enhancements can be made by increasing spacing, pitch, depth, moisture, loops, etc.
Generally required for larger GSHP systems, where loads must be more precisely met in order to eliminate un-necessary redundancy or oversizing expense.
Soil sampling and analysis or in situ conductivity testing is usually specified.
With 4,100 vertical bore holes at 400 ft. deep each, Ball State University will soon have the largest ground source heat pump system in the country. More Facts: http://cms.bsu.edu/About/Geothermal.aspx
More Facts: http://cms.bsu.edu/About/Geothermal.aspx
More than 40 buildings on the 660-acre Ball State University campus will be converted to geothermal heating and cooling. It is slated for completion in 2013.
Closed-Loop Vertically Bored GHEX Even a rock bored system can pay off…
1¼“ U-Bend pipe being reeled directly into 320’ vertical hole
Spreader clips being clamped to U-bend & tremie pipe assy.
Closed-Loop Vertically Bored GHEX …but metal casings required down to rock!
After grouting, U-bend pipes are pressure tested and header trench is excavated…
Casings are cut & removed from trench (orphan spreader clip visible in exploded view).
Proper grouting provides sanitary protection of water supply, preserves hydraulic characteristics of artesian formations…and improves heat transfer!
Pressure tremie grouting is required for each bore hole using appropriate grout mixing and pump equipment.
Cross section of cement grouted U-Bend bore without spreader clips.
In rock formations neat cement is mixed with fine sand or other additive to enhance grout.
In soil formations use a high-solids “Western” Bentonite mixture—NOT Mud Drilling Grout.
Optional Configurations for 3 Ton Vertically Bored Closed-Loop GHEX
• One loop circuit per Ton of nominal GSHP capacity (this will vary by design!)
• 200’ X ¾“ U-Bend HDPE pipe/bore hole
• Thermally enhanced high-solids Western Bentonite or Port-land Cement Grout
• You must use Propy-lene Glycol in MN
• In MN…bore holes must be fully cased in any unconsolida-ted overburden over drilled rock!
Long Term Thermal Effects of GHEX on Deep Soil & Rock Formations
B = bore hole spacing
H = bore hole depth
g = temp resistance
t1 = time constant
= H2/9a a = diffusivity = k / (r c) a = 0.6 ft2/day H = 200 ft
6 months
20 years
Adjusted Bore Length for Current 5T System (Also applies to Horizontally Drilled GHEX Designs!)
Vertically Bored GHEX
ADVANTAGES:
Requires least amount of site surface area
Minimal excavation required (header only)
Versatility in pipe configuration & placement (may utilize small yards, parking lots, etc.)
Higher, more stable deep earth temperatures (Duluth Complex 46°F to 48°F)
Vertically Bored GHEX
DISADVANTAGES:
Cost is generally higher compared to other GHEX loop options
Might require adding expensive steel casings in unconsolidated overburden when boring into rock formations (must use in Minnesota!)
Propylene Glycol is usually specified instead of better performing Methanol (Minnesota!)
Drilling equipment access can be limited
Drilling fluid & cuttings can make a huge mess
There is often more science involved in grouting than drilling—local contractors may be limited
Horizontal Directional Drilling
HDD Machine Drilling 150 Ton GHEX for a
Minnesota School
Horizontal Directional Drilling
Drill Bit Emerging from Borehole
U-Bend & Tremie Attached to Reamer
HDD Machine Pulls Drill Stem As Pipe Reels Off Far End
Tremie Pulled Back to Grout Borehole
Pipe Tails In Header Trench
U-bend Pipe End –
Horizontal Directional Drilling
Drill Bit Emerging from Borehole
U-bend & Tremie Attached to Reamer
Header Pipe Attached to Reamer
U-bend Being Pulled from Reel
Horizontal Directional Drilling
Pulled Pipe Tails Ready for Header
Excavation
Horizontal Directional Drilling
Header Excavation Reveals “Fanned”
Loop Configuration
The Importance of Responsible Grouting (Big Cost Difference—about 70’ per Bore Length!)
Good Grout Conductivity = 1.2 5 X 263 ft. Horz. Bore Length
Poor Grout Conductivity = 0.4 5 X 330 ft. Horz. Bore Length
Optimum Grout Conductivity? By increasing “design” grout conductivity,
required pipe and bore lengths will decrease
Grout conductivity need not exceed that of the formation (but it is still better to come close)
Grout (and grout application) costs increase as conductivity and enhancements are increased
Cheapest grouts start at about 0.40 Btu/hr/ft °F
It has been effectively demonstrated that Grout Conductivity Benefit per Reduced Bore Length Cost drops off at about an “88” (0.88) grout.
• One loop circuit per Ton of nominal GSHP capacity (will vary by design!)
• 225’ X ¾“ U-Bend HDPE pipe/bore hole
• Minimum “average” recommended pipe depth & spacing is 15’
• Thermally enhanced
high-solids Western Bentonite Grout is highly advised
• Methanol is presently acceptable; Propylene Glycol may soon be required
HDD loops may be loosely described as “a vertical loop set on its side” (bottom tier limited to 45’ depth)
Horizontally Bored GHEX
ADVANTAGES:
Versatility in pipe configuration & placement: possible beneath buildings, parking lots, play fields, trees & landscaped areas, other obstacles
Tier-level “layering” is possible (from 45’ depth)
Minimal disturbance to site—excavation can even be limited to small consolidated area using “fan” configuration and “close header”
Cost generally lower than vertically bored GHEX (may also be competitive with some horizontally trenched applications—but also consider grout!)
Horizontally Bored GHEX
DISADVANTAGES:
Limited to unconsolidated (soil) formations
Required HDD bore lengths and spacing may exceed property dimensions and/or setbacks
Boulders or other obstructions may hinder boring, damage equipment, or even alter design.
HDD grouting is currently unregulated and often ignored—or even purposely eliminated just to lower cost!
Coming under closer scrutiny by state health regulators (particularly “slant and plant” drilling)
QUESTIONS?
Horizontally Trenched (or Excavated) GHEX
What is wrong with this picture?
What is wrong with this picture? Nothing is wrong with this picture!
SOURCE: Closed-Loop Geothermal Systems Slinky® Installation Guide, Rural Electric Research (RER) Project 86-1, IGSHPA 1996.
Correction Table for Different Trench Spacing
Selected pipe and trench spacing should not change GSHP design efficiency—just pipe length, excavation footprint…and cost!
(5’ Scc)
2’
How did we ever get the “slinky”?
The world’s “first” geothermal heat pump system—probably a single pipe in one long trench...
Eventually modified to con-serve space and installation expense…
Current 5T System Single 1¼“ Pipe Laying
= 4,721 ft.
Trench = 4,721 ft.
Current 5T System Two 1¼“ Pipe Standing
= 6,063 ft.
Trench = 3,032 ft.
How did we ever get the “slinky”?
Then advanced to multiple parallel circuits using a single common header...
Current 5T System Four 1” Pipe Standing
= 9,491 ft.
Two Trenches @ 1,186 ft.
How did we ever get the “slinky”?
Then advanced to multiple parallel circuits using a single common header...
Until someone figured out yet another way to conserve time, space, and resources!
(But not without new challenges—clay slurry shown being applied to fill voids inside trench)
Current 5T System Four 1” Pipe Standing
= 9,491 ft.
Two Trenches @ 1,186 ft.
“Laying Slinky” Configurations for Colder Climates
Closer Spacing = longer pipe requirement, smaller consolidated excavation footprint (500 ft2/T)
Wider Spacing = shorter pipe requirement, expanded excava-tion footprint (1,000 ft2/T)
Pipe coils are rolled out individually down entire length of trench…and back.
Example of excavated 8 Ton “Laying Racetrack” configuration (850 ft2/T)
“Laying Racetrack” Configuration is still an option.
Standard or “Racetrack” Configuration for Cold Climate GHEX Applications
• One loop circuit per Ton of nominal GSHP capacity (as sized to peak heating load!)
• 800’ X ¾“ HDPE pipe per coil
• Return loops are spaced 12” apart (minimum)
• Excavation Footprint ≈ 2’ X 400’ per coil @ approx. 8’ depth
Typical Slinky GHEX Configuration for Cold Climate Geothermal Applications
• One loop circuit per Ton of nominal GSHP capacity (as sized to peak heating load!)
• 800’ X ¾“ HDPE pipe per slinky coil
• Each coil is 36” dia-meter and over-lapped every 18” (equivalent to 8 pipes laying)
• Slinky coil = 95’
• Excavation Footprint ≈ 5’ X 100’ per coil @ approx. 8’ depth
How Moisture Improves Thermal Conductivity of Soil
At complete dryness the heat flow passes mainly through the grains, but has to bridge the air-filled gaps between the grains around their contact points.
air
Water
film
Water
Water
At very low water contents the soil particles are covered by thin absorbed water layers
The thickness of these layers increases with increasing water content. At a certain Xw liquid rings start to form around the contact points between the grains; they show a curved air-water interface.
From this point on the thermal conductivity increases rapidly with increasing Xw, until the rings almost completely fill the original gap. When Xw increases still further the complete pores are filled with water, up to saturation. This is reflected by the slower increase of k with Xw.
Soaker Systems A storm water or gray water drain tile piping system can be laid-in on top of a horizontal GHEX to enhance moisture in dry soils. (This approach may also be considered where driveways or paved parking lots are planned. Generally such systems are also buried much deeper.)
ADVANTAGES:
Generally costs less than either vertical or horizontal drilling options
Excavation process remains fairly straight-forward—local (even on-site) operators can usually be used
Seasonal loop recovery tends to be more forgiving close to the surface where sun and rain have influence (esp. “heat only” GSHP systems)
Often the most practical, as well as economical, option in challenging or unpredictable geology
Horizontally Trenched GHEX
DISADVANTAGES:
Requires the largest area of available space: Total excavation area = entire GHEX footprint (and then some!)
Arguably the largest impact with least amount of flexibility in loop configuration and placement
Cannot be placed beneath structures, driveways, septics, parking lots, etc.—with a few exceptions
Other buried services can be affected if crossed
More vulnerable to damage during installation
Horizontally Trenched GHEX
Pond & Lake Loops
Pond Heat Exchangers Offer an attractive alternative to buried loops
Pond Heat Exchangers combine exceptional GSHP system performance…
With an aesthetic component you just can’t get from a con-ventional earth loop.
Pond Heat Exchangers Offer an attractive alternative to buried loops
Pond Heat Exchangers combine exceptional GSHP system performance…
With an aesthetic component you just can’t get from a con-ventional earth loop.
Floated Geo-mat Application PHEX is constructed on shore, floated & sunk.
Conventional 18” pitch slink-ies are zip-tied together and covered with chain link mesh…
This 8 Ton GSHP application operates consistently at 36°F EWT throughout winter.
Hybrid Pond Heat Exchangers Hybrid Pond/GHEX for potentially low water
Conventional slinky GHEX can often be laid directly in pond basin (at standard 8’ depth)…
Cover it with 2’ of clean gravel before flooding and it may be more forgiving during drought.
Many Other PHEX Design Options More condensed arrays…require loose spacing
Mountain melt water basin in Steamboat Springs, CO, used for Geo’…and trophy trout!
This 55 Ton application utilizes 4 X 10 loops using 500’ coils of 1” HDPE—33 Tons added later.
Pond Heat Exchangers Cages combine durability with on-site mobility
The 700,000 sq. ft. building complex of Great River Medical Center in Burlington, IA, is served by 800 heat pumps and a 1,500 Ton loop system with 82 miles of pipe in a 15 acre lake! More Facts: http://www.alliantenergygeothermal.com/GeothermalInAction/CommercialGovernmentBuildings/000436
ADVANTAGES:
Can cost more than a conventional buried loop if pond must be excavated—but less if it is already existing!
Better expected GSHP performance & efficiency: loop temperatures between 33°F and 40°F
Minimal disturbance to site—excavation generally limited to a single S/R header trench to house
Aesthetic value
Pond & Lake Loops
DISADVANTAGES:
Potential for loop freeze-up and failure due to drop in pond level/volume (ways to minimize)
Exposures—pipe needs protection from boat anchors, fish hooks…and beavers!
Permits—wetland permit may be required for excavated pond; state water use permit is always required for “lake energy exchanger” placement and/or removal within any MN/WI public water
No dependable, well established design criteria for pond loops—nominal PHEX configurations are most typical
Pond & Lake Loops
Open Loops
Open Loop (Pump-and-Dump) Systems Far less costly—but might pose other concerns
Instead of a buried closed loop GHEX, domestic water from house is simply pumped through the GSHP coil then discharged somewhere outside. (Pictured is a simple shallow drain tile in sand.)
Open Loop (Pump-and-Dump) Systems Leaving water temperature is tuned to 37°F.
An open loop flow control assembly typically includes, filter strainer, P/T ports, back-flush bibs, solenoid valve, and flow meter. (Pictured is an assembly that is “presuperheater” capable.)
Open Loop systems allow for the same amount of flexibility in GSHP design.
• Lowest safe entering water temperature (EWT) limit = 43°F
• Flow requirements scale upward from 4 to 14 GPM (approx. 3 to 8 Ton GSHP) for residential sizing
• LWT is tuned to 37°F regardless of EWT (determines ultimate system flow rate)
• Up to 10K GPD & 1M GPY allowed without permit in MN
How much is a “lot” of water?
Calculate number of gallons in 1" rainfall on 1 Acre of land:
43,560 sq.ft./Acre 144 sq.in./sq.ft **BUT** 1,728 cu.in./sq.ft. 43,560 X 1,728 = 75,271,680 cu.in./Acre 0.00432900433 gals./cu.in. 75,271,680 X 0.0043290 = 325,851.1 gals.
325,851 gallons fall on 1 Acre of land for every 1" of rainfall. On a controlled open loop, a 5 Ton cold climate GSHP discharges approximately twice that amount (650,000) gallons in one year.
Open Loop Reinjection Well Options Where surface discharge opportunities are limited
Diffusion/Reinjection Standing Column
Figure 1.2a. Ground Water Open-Loop GSHP System Figure 1.2b. Standing Column Open-Loop GSHP System
Coaxial “Standing Column” Reinjection Wells Limited to rock formations—Must be precisely engineered
ADVANTAGES:
Pump-and-Dump design, using existing domestic well, is least expensive loop option available
Requires minimal disturbance to site—excavation generally limited to single discharge pipe trench
Higher expected GSHP performance & efficiency: loop temperatures generally fall between 43°F and 48°F in Duluth Complex
Heat pump sizing can often be scaled back by a nominal Ton—making many forced-air retrofits much easier
Installation is frequently “plug and play”
Open Loops
DISADVANTAGES:
Higher domestic water usage can potentially affect aquifer
At the mercy of the well—water temperature and recovery rate must remain sufficient and stable
Water quality can also be a factor—routine back-flushing of the GSHP water coil is generally recommended
Metered city water can be cost prohibitive
Site discharge opportunities may be limited (no septics or sanitary sewers)
Reinjection wells are inherently risky in lower temperature formations like Duluth Complex—they are also starting to present other concerns, particularly relating to well water contamination.
Open Loops
QUESTIONS?
END
RESOURCES:
Minnesota Geothermal Heat Pump Association www.MNGHPA.org
Wisconsin Geothermal Association www.wisgeo.org
International Ground Source Heat Pump Association www.igshpa.okstate.edu
GeoExchange www.GeoExchange.org
Association of Energy Engineers www.aeecenter.org
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